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Lateral view of a Male Baetis (Baetidae) (Blue-Winged Olive) Mayfly Dun from Mystery Creek #43 in New York
Blue-winged Olives

Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.

Dorsal view of a Limnephilidae (Giant Sedges) Caddisfly Larva from the Yakima River in Washington
This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive ID.
27" brown trout, my largest ever. It was the sub-dominant fish in its pool. After this, I hooked the bigger one, but I couldn't land it.
Troutnut is a project started in 2003 by salmonid ecologist Jason "Troutnut" Neuswanger to help anglers and fly tyers unabashedly embrace the entomological side of the sport. Learn more about Troutnut or support the project for an enhanced experience here.

Mayfly Genus Isonychia (Slate Drakes)

Sporadic hatches are rarely as outstanding as those of Isonychia. On streams with good populations, they are reliably hatching in light numbers, here and there, for most of the evening through most of the mid- to late season.

The spinners, and occasionally the duns, produce more concentrated action, but the real value of the Isonychia hatch is its duration and the size of the flies; large trout become ever watchful for them, even when they aren't emerging.

All the species of Isonychia are similar in appearance and behavior.

Where & when

Preferred waters: Best in freestone streams

Isonychia is mostly important in the East and Midwest, where all the action is provided by Isonychia bicolor. The former species Isonychia sadleri and Isonychia harperi were recently discovered to be synonyms of bicolor. In the West, Isonychia hatches are not very important, and they may be attributed to several minor species, especially Isonychia velma.

Some people say the Isonychia species are multibrooded, but this is not technically correct, even though their pattern of emergence is similar. In true multibrooded mayflies like the Baetidae, the flies emerging in midsummer or Fall are the offspring of the earlier hatch from the same year. In Isonychia, the Fall emergers are offspring from the previous Fall. They are present as half-grown nymphs when the first of their generation emerge.

In 120 records from GBIF, adults of this genus have mostly been collected during June (38%), July (31%), August (16%), and September (6%).

In 42 records from GBIF, this genus has been collected at elevations ranging from 180 to 5407 ft, with an average (median) of 3018 ft.

Genus Range

Hatching behavior

Isonychia duns may emerge on the surface or by crawling out onto shore. This behavior can vary within a single species, and it seems to depend on geographic location and the weather conditions.

Spinner behavior

Time of day: Dusk

Habitat: Riffles

Isonychia duns molt into spinners within a couple days of hatching, and when they return they provide much more concentrated action than the emergence. They mate in swarms twenty to thirty feet in the air, and the females drop their eggs from high above the water before they join the males in falling spent. Some conflicting accounts say the females oviposit by descending to dip the tips of their abdomens into the water over and over.

I experienced a good Isonychia bicolor year on one northern Wisconsin river during which there were many duns for up to a month, but I never saw any spinners despite fishing appropriate riffles dozens of times at dusk. I have not solved this mystery, but there is more to timing these events than has been so far discovered.

Nymph biology

Diet: Mostly plankton; sometimes other aquatic insects

Current speed: Medium to Fast

Substrate: Boulders and gravel

Environmental tolerance: Quite tolerant of pollution and marginal temperature

Isonychia nymphs are among of the fastest-swimming mayflies in the world. They can power their way through fast riffles with ease, and their imitations should be fished with fast twitches.

They are very unusual mayfly nymphs for three reasons:

  • They have tufts of setae on the insides of their forelegs which they hold up in the current as nets to filter out plankon for food.
  • Despite their superb plankton-feeding capabilities, they are also among the few types of predatory mayflies. They may feed on midge and caddis larvae and smaller mayfly nymphs.

  • They are unique among mayflies in that they have extra tuft-shaped gills at the base of their fore legs, a structure normally found in stoneflies.

Specimens of the Mayfly Genus Isonychia

5 Female Duns
1 Male Spinner
2 Female Spinners
13 Nymphs

5 Streamside Pictures of Isonychia Mayflies:

Discussions of Isonychia

Are Isonychia mayflies technically multibrooded?
4 replies
Posted by Troutnut on Jul 22, 2006
Last reply on Apr 17, 2009 by GONZO
Here's what I've written in my article on Isonychia about their hatching:

Some Isonychia species are multibrooded, but not in the same way as most other multibrooded mayflies like the Baetidae. In those species, the flies emerging in midsummer or Fall are the offspring of the earlier hatch from the same year. In Isonychia, the Fall emergers are offspring from the previous Fall. They are present as half-grown nymphs when the first of their generation emerge. Although Isonychia broods have distinct peaks, some may be found on the water at any time in between.

I'm curious if they can really be called multibrooded or not, since they don't produce more than one generation per year (as far as I know). They simply have distinct populations within the same generation which emerge at different times during the year. Does that count?

All my books are packed up in boxes right now so I don't have a technical definition of the term handy.
Penns Creek Slate Draker's
4 replies
Posted by Jsell925 on Jul 17, 2007
Last reply on Sep 23, 2007 by Shawnny3
Penns is one of the few places where a #10 iso will nail em' all year long
1 replies
Posted by JMV on Sep 21, 2006 in the species Isonychia bicolor
Last reply on Sep 21, 2006 by Troutnut
Great site, I'm an Iso. fanatic... JM

Start a Discussion of Isonychia


Mayfly Genus Isonychia (Slate Drakes)

Genus Range
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